I'd suggest getting a small...
I am saying this for more of personal thing but since your FC is just inventory space, taking Medium plots away from FCs that have 50-100 individual people is kinda...
It's just a personal thing. I have a house on my server (Medium/Mist/FC house)...so I don't really care but you may piss people off.
There's no reason one person needs a Medium. Smalls are just fine. And yes, you and your 7 alts ARE one person.